Low cost color copiers are able to effectively handle the reproduction of color documents and especially, continuous tone color image data. However, due to a mixture of image scanner inaccuracies and non-linearities in color mapping, color copiers tend to cause text to be rendered with either too little or too much color. As a result, black text can be reproduced with lines that are too thin or evidence a shade of grey, rather than dark black.
The non-linearities in color mapping arise from chromatic misalignments between color planes which may, at times, be on the order of a dimension of a pixel. Such misalignment affects the sharpness of text boundary, introduces extraneous colored regions around the text and makes black discrimination more difficult. A further effect is "optical flare" which results from a combination of limited optical resolution in the copier's optics and the effects on the scanner of stray light. Optical flare can affect different print color channels disproportionately; can skew the color of the black text causing it to seem to shift towards green; and can lighten the intensity of the black text. The width of the text can also be affected.
Many image processors today use image convolution to improve image presentations. For instance, a form of image convolution is utilized when dither patterns are applied to a scaled image to improve its grey scale rendering. Image convolution, when applied to a pixel-based raster image, involves a logical superimposition of an N.times.M matrix window of values over an area of the image, in "alignment" with pixels of the image. The matrix window, at each position, manifests a value which is logically combined (e.g., by multiplication of the values) with an aligned image pixel, to arrive at a resultant product. The products are then added and substituted for a central image pixel within the matrix window so as to modify the central pixel in accordance with the logical operation performed using the matrix window. The matrix window is then shifted across the image so as to subject every pixel to the convolution process.
Depending upon the size of the matrix window and the number of values in the matrix window, the image convolution process can be time-consuming and processing intensive. Nevertheless, the prior art has employed image convolution in attempting to improve (or "sharpen") an image's boundaries. The basic prior art approach to such print sharpening is called "high-boost filtering". Such filtering enables the high frequency components of an image to be isolated and added back into the original image to make the edges of the original image both sharper and darker. More specifically, the prior art has subjected a pixel image to an initial low-pass filtering action to derive a "blurred" version of the original image. That blurred image is then subtracted from the original image to accomplish an "unsharp" masking. This action identifies the high frequency components of the original image.
More specifically, since an image is represented by discrete pixel values, the term "frequency" refers to the spatial frequencies in "cycles per pixel" units. As in one dimensional signal processing, pixel edge transitions have many frequency components and image regions which show large rates of intensity change have many high-frequency components. Hence, if areas of an image can be identified which exhibit high frequency components, those portions of the image can be sharpened by "high frequency boosting" through the alteration of the pixels where the high frequency components reside.
Even though the concept of high boost filtering to enhance image edges is known; the use of matrix windows, the shifting thereof across high-definition pixel images of (e.g. 300-600 dpi), and the resultant processing of the results are computationally intensive and time-consuming.
Accordingly, it is an object of this invention to provide improved apparatus for high boost filtering of a pixel image so as to enhance image edges.
It is another object of this invention to provide a circuit which implements a high-boost filtering procedure, wherein matrix calculations are minimized.